Phenolic resins are well known for their durability, heat-resistance and water-resistance. These and other features make them desirable for use as adhesives and coatings for cellulosic materials or as coatings for metals. However, they are brittle and do not perform well as adhesives for metals unless modified by substantial adhesion of resins that have better adhesion to metals, such as epoxies, or more flexible resins, such as nitrile or chloroprene rubber, or certain vinyl resins. Most phenolic resins are applied in solution in an organic solvent; however, some low molecular weight phenols are water soluble or dispersible.
Ethylene copolymers or interpolymers are flexible thermoplastic resins whose properties generally resemble polyethylene. Certain of these copolymers, such as those of ethylene and a carboxylic acid such as acrylic acid have a superior ability of adhesion to various substrates. Such copolymers are often used as hot-melt coatings or adhesives, either alone or compounded with other thermoplastic resins, waxes, etc. A unique property of certain of the ethylene-carboxylic acid copolymers with high acid content is their ability to dissolve in aqueous alkali to give colloidal solutions by a reaction similar to saponification.
I have found that a solution or dispersion of ethylene-carboxylic acid copolymers in a volatile alkali blended with solutions of phenolic resins can give a composition that may be applied and dried as a coating, which may be first reactivated (or fused to becomes tacky) and then thermoset by heating, thus yielding a strong, tough, flexible material that has excellent adhesion to many substrates and is useful as a primer, adhesive, or coating. Modifications of this basic composition have been developed by addition of other materials such as other resins, fillers, pigments or dyes, solvents, surfactants, dispersants, or cross-linking agents, in order to improve such properties as stability, coatability and color.
One useful application of this composition is as a primer for use in conjunction with certain thermoplastic adhesive and coating meterials, especially those based on ethylene copolymers, terpolymers, or ionomers, with and without modifiers such as coal-tar pitch (U.S. Pat. No. 3,361,692).
When bonded to fibrous or porous substrates, such as wood, paper, asbestos-cement, etc., these thermoplastic adhesive compositions are too viscous, even at temperatures up to 500.degree.F (260.degree.C), to flow around the fibers or into pores to any considerable depth. While the adhesive may adhere or bond to the superficial fibrous or porous surface, it does not penetrate deeply. Such superficial surfaces are often weaker than the bulk of the substrate because they may be damaged by manufacturing operations such as cutting or abrading. Thus, bonds to such substrates may be relatively weak. Furthermore, bonds between metal and such fibrous or porous substrates are often subjected to severe stresses when subjected to theremal or humidity changes because of great differences in expansion characteristics of the adherends.
U.S. Pat. No. 3,211,804 teaches a method for making an adhesive or coating in which is added phenolic to olefin polymers not in an emulsion, nor in a water base dispersion, but merely blends the two resins in order to improve their characteristics such as heat resistance, clarity, solvent resistance and adhesion. However, the flow index is extremely low which indicates that the composition is cross-linked resulting in high viscosity and a loss of thermoplasticity. While my composition in the applied and cured condition has these same characteristics, it has the ability to flow readily, wet and adhere to the surfaces to be joined, before cross-linking occurs. Thus, my composition can be easily applied to surfaces before curing.
Very few adhesives are capable of withstanding cycling tests such as the American Plywood Association's soak-dry, hot-water-dry, or soak-freeze-dry cycling tests with a metal overlay on plywood. To pass such a test, it is necessary for an adhesive to be very flexible and tough and its adhesion to both the metal and the fibrous or porous substrate must be very strong and moisture resistant.
A dilute solution of my composition, applied as a thin primer coating to the porous substrates, penetrates and impregnates the surface fibers or pores and, when dried, binds and toughens the surface of the substrate. When the thermoplastic adhesive is applied with heat and pressure sufficient to cure the primer on the surface, the resultant adhesive bond is much stronger, more moisture-resistant and more heat resistant than that without the primer.
A dilute solution of my composition is also advantageous when similarly applied to metal surfaces as a primer in conjunction with the above mentioned thermoplastic adhesive for use as adhesives or protective coatings. The low viscosity, low surface tension and soap-like properties of my composition make it possible for the primer to wet the metal surfaces, even when the surfaces are contaminated with a small amount of oil or grease. The composition thus functions as a cleaner for the metal. The dried primer coating reactivates and cures when heat and pressure are applied to bond the thermoplastic to the metal and gives a bond between metal and the thermoplastic adhesive or coating that is superior to that obtained without the primer under similar conditions. It is possible to obtain satisfactory bonds at lower temperatures and shorter time-at-temperature with such primed metal. The bonds with thermoplastic composition on the primed metal are stronger, more heat resistant and more durable on long time exposure to moisture than bonds made without the primer. This improved performance with my primer is important when the thermoplastic composition is used either as an adhesive or as a protective coating on metal or other substrates for severe applications such as a coating for the insides of culverts.
Adhesives are used for bonding metal faces to cores, such as plywood, paper or metal honeycomb, fiberboard, etc., as sandwich structures or metal overlays for architectural, automotive, and other applications. My composition is an excellent adhesive for such bonding. In such applications, it should be applied to the adherends as a more concentrated solution than that used as a primer, in order to build up an adequate thickness of glue line. No primer is necessary because this compound wets and adheres to the metal and penetrates the surface of porous core materials. After drying, the metal and the core are assembled by applying heat and pressure to reactivate and cure the adhesive. The bonds are strong both in shear and peel strength, and resistant to moisture and heat.
This adhesive system has several advantages over competitive adhesives such as epoxies, rubber-phenolics, or thermoplastics. It may be preapplied and dried on the adherends from an aqueous solution (without fire hazard), the components can be assembled dry, and, when cured under heat and pressure, the adhesive is theremoset so that pressure may be released and the panel handled while still hot without delamination (as would occur with thermoplastic adhesives).
In addition to the use of my compositions as primers or adhesives, I have also found that they may be used themselves as heat curing paints, enamels, or protective coatings. For such applications it is advantageous to use more concentrated solutions (compared to primers) in order to apply coatings of sufficient thickness to provide the required protection. To the aqueous basic composition of ethylene copolymers and phenolic resins it may be desirable to add modifiers such as fillers, pigments, solvents, or other resins in order to obtain the desired coating characteristics. These coating compositions may be applied to metal or other substrates by spraying, roll-coating, or other application processes as commonly used for other paints or enamels. They should then be dried and baked to fully thermoset the resin composition. Such coatings have been found to be tough, flexible, abrasion resistant, and excellent protection from corrosion under severe moisture conditions.
Another specialized use of my composition is as a coating or "core plate" for use on steel sheets for electromagnetic cores such as those in transformers, generators, or motors. The cores of transformers and other electromagnetic structures are constructed of laminations to restrict the flow of eddy currents and hereby reduce this component of the electromagnetic core loss. For this to be successful, however, it is necessary that each lamination be insulated from the other so that the eddy currents will not flow readily from one lamination to another. In some cases, insulation, in addition to that from normal surface oxide, is supplied by coating the sheets or laminations with a thin coating of varnish or organic "core plate", which has good electrical resistance and which is capable of maintaining this resistance under normal operating temperatures and pressures. When used in oil-immersed transformer cores, the core plate must not dissolve or react with the oil because the insulating properties would be destroyed. The usual method of applying this coating is to pass the sheets or laminations through rolls coated with the composition, and then through a combination drying and baking oven. The thickness and uniformity of the coating must be controlled to obtain adequate insulating properties. The coating should not be too thick, however, because this will, in effect, reduce the amount of steel in a core stack. Baking temperatures differ, depending on the composition and type of coating used. The organic varnishes require care in baking, since it is essential to volatilize off most of the vehicle and cure the resin to obtain a coating free from tackiness. Under-baking results in a soft, tacky or thermoplastic coating which will not have satisfactory insulating properties under the pressure and temperature conditions present in the core during service. Care also must be taken to prevent over-baking because the varnish will become carbonized and the insulation characteristics of the coating will be thereby impaired.
Presently used organic varnish core plate requires the use of flammable organic solvents which are too hazardous for application under preferred plant conditions. There is a clear need in the art for a coating material which may be applied to the steel laminate but which is not flammable. This new material must also meet several other requirements, e.g., it must have good insulating properties (0.50 amperes maximum at 300 psi and 150.degree.C ASTM A344-68), is should have an obvious color, it must be inert to oil, and it should be amenable to coating in dry thicknesses ranging from about 0.15 mil to 0.03 mil (3.6 to 0.7 microns). Further, the coating should act as a lubricant for the die during punching of the core forms.
My compositions can meet all the above requirements for a coating of electromagnetic steels, when applied as a thin, waterbased coating, dried, and baked at sufficient temperature and time to thermoset the coating as completely as possible without deterioration.
This "core plate" coating composition may also be used as an adhesive for bonding the sheets or lamillae of the electromagnetic cores by a different processing method. After applying the coating composition, it is dried at a temperature and time sufficient only to evaporate the volatile materials but not enough to cure the coating. This uncured coating is tough and sufficiently adherent to withstand the fabrication and assembly of the core. After the core with the uncured coating is clamped together, it is baked to reactivate and cure the coating, thus causing the sheets to adhere to one another. This process of adhesive bonding of electromagnetic cores may be of value in reducing the need for mechanical fasteners to hold the core laminations together and for reducing the vibration and noise that occurs in unbonded cores because of magnetostriction.
The vibration damping properties of my adhesive compositions for bonding laminates can be greatly improved by replacing about 20 to 80 percent of the ethylene-acrylic acid copolymer with ethylene-vinyl acetate copolymers or terpolymers which soften and improve the viscoelastic damping properties of the adhesive compound. Dispersions of these terpolymers and copolymers are available as Elvax D dispersions from E. I. duPont de Nemours and Company. Bonded metal laminates with these modified damping adhesive compositions have been found to be very effective in damping vibration and sound. They should be useful not only in reducing the noise and vibration of electromagnetic cores as mentioned above, but also for vibration and sound damping of metal components of machines, appliances, furniture, etc.
Thus, among the objects of this invention are the following:
1. To provide a thermosetting adhesive and coating composition that may be applied from an aqueous dispersion. PA1 2. To provide a material for use as a primer on various substrates to improve the bonds with certain thermoplastic adhesives and coatings. PA1 3. To provide an adhesive material suitable for bonding metal to various substrates such as metal, paper, wood, ceramics, etc., especially for bonding laminates, or sandwich structures. PA1 4. To provide a material for use as a thermosetting coating, paint, or enamel for the protection of surfaces of metals and other substrates. PA1 5. To provide a material suitable for applying a thin insulating and lubricating coating or "core plate" to steel sheets for use in laminated electromagnetic cores. PA1 6. To provide an adhesive material with good viscoelastic properties suitable for bonding metal laminates to be used for vibration or sound damping applications.
These and other objects will become apparent from the following detailed specification.